Example of Open In Power Supply (B+) Circuit

Bad Battery + Pattern (Typical Failure):

BACKGROUND AND APPLICATION INFORMATION:
This waveform is presented for reference to show what a failure looks like on the display of the Digital Storage Oscilloscope (DSO). In the example above, an open in the B+, or battery supply voltage, to the Powertrain Control Module (PCM) is the culprit. A bad wiring harness connector was the cause. When comparing this example to the "POWER SUPPLY (B+) TEST" example, it is easy to see why amplitude is the Critical Dimension for DC signals. The downward spikes indicate an open in the power supply circuit.

CONNECT AND SETUP THE DIGITAL STORAGE OSCILLOSCOPE (DSO):
1. Connect the "COM" probe to the negative battery post or the engine block.
2. Connect the "CH1" probe to the B+ circuit to be tested.
3. Set the voltage scaling to 5 V/div.
4. Set the time base to 500 ms/div (1/2 second per division).
5. Select "GND" coupling and position the trace on the second division line from the bottom of the DSO display.
6. After ground position is set, select "DC" input coupling.
7. Select "SPIKE DETECT" or "MIN/MAX" acquire mode.
8. "TRIGGER" mode should be set automatically (Roll Mode).

EXERCISE THE SENSOR, SWITCH, OR CIRCUIT UNDER TEST:
Make sure power is switched on in the circuit so that the sensor, device or circuit is operational. Exercise the sensor, device or circuit while paying particular attention to the amplitude of the signal. It should stay in a predetermined voltage range for a given condition. In most cases, the amplitude of the waveform should stay at B+, or battery voltage, when the circuit is on, and go to zero volts when the circuit is off or the switch is activated.


EXPECT THIS WAVEFORM RESULT:
If the circuit under test is a B+ power supply, the waveform's amplitude should not change with respect to time. If the circuit under test is a switch or other operational device, then the waveform's amplitude should only change if the switch or device is being operated. Possible violations of the "Critical Dimension" rule to watch for are spikes to ground, as in the above example (meaning there is an open circuit in the power side, or a voltage short to ground), upward spikes (meaning possible open in ground side), or out of range voltage (either too low or too high).

The 5 Critical Dimensions

FOR MORE INFORMATION
Description of Automotive Signals
Diagnosis Using A Labscope
Labscope Quick Reference Appendix